Abstract:The spin-dependent neutron-proton total cross-section differences Δσ L and Δσ T have been measured between E n =5 and 20 MeV in longitudinal and transverse nucleon spin orientations. From these data the 3 S 1 - 3 D 1 mixing parameter ε 1 , which characterizes the nucleon-nucleon tensor force at low and intermediate energies, was determined in a model-insensitive way. In combination with measurements at higher energies, our values for ε 1 support a nucleon-nucleon tensor interaction that is stronger than predicted by all modern high-precision nucleon-nucleon potential models and phase-shift analyses.